Keatite; II, Hydrothermal synthesis from silica-glass

The metastable, microcrystalline silica phase k has been synthesized from solid silica-glass rods and from granular high-purity (SiO (sub 2) 99.99 wt.%) silica-glass at 400 degrees C, 350 bars with a run time of 1-2 days in 0.01 N NaOH/H (sub 2) O solution. Variations in Na (super +) concentration in the solution lead to the growth sequence quartz-fibrous keatite-disordered cristobalite C. Keatite coexists with quartz and/or cristobalite, with sharp microstructural boundaries. As the Na (super +) is incorporated in keatite, the pH of the solution changes from 12 to 9, and in this less alkaline solution keatite transforms to quartz in a few hours, accounting for the low yield under hydrothermal conditions.